Lately, in paint field, especially in the field of automobile coating, decreasing coating step has been strongly required to attain resource saving, cost-saving and decreasing damage to environment (VOC and HAPs, etc.). Specifically, a method for forming a coating film with intermediate-coating-less (two-layer coating system) is desired in which top coating is applied directly on an electrodeposition primer coating. The two-layer coating system reduces number of coating steps compared with three layer coating having an electrodeposition primer coating, an intermediate coating and a top coating. The two-layer coating systems should also have composed of excellent appearance, adhesiveness to top coating, weather resistance and corrosion resistance equal to the conventional three layer coating.
Regarding electrodeposition coating by way of the intermediate-coating-less system, Japanese Patent Kokoku Publication No. 33069/1990 discloses an electrocoating composition which forms two-layered coating with thick film thickness type. The invention is directed to a composition which contains a cationic acrylic resin having a softening point of 80° C. or more and a cationic phenol type epoxy resin having a softening point of 75° C. or less at a weight ratio of 1 to 30:1. It is described that the coating film formed from this composition has a two layer structure composed of an epoxy type underlayer having good corrosion resistance and an acrylic type upper layer having good weather resistance.
Japanese Patent Kokoku Publication No. 99652/1994 demonstrates that a cured multi layered electrodeposition coating film can be formed from an epoxy type cation electrodepositable resin and a non-ionic film-forming resin, which have particular surface tension.
Japanese Patent Kokai Publications Nos. 333528/1996 and 292131/1998 also disclose a cured multi layered coating film which is formed from a composition containing an amine-modified epoxy type cation resin, a (self-crosslinking) acrylic type cationic resin having lower solubility parameter than the former resin, and a block polyisocyanate curing agent.
Japanese Patent Kokai Publication No. 345394/2000 discloses that an electrodeposition coating composition containing two different resins that are incompatible with each other and have solubility parameter difference of not less than 1 is electrically coated to form an electrodeposited coating with complete layer separation which provides with corrosion resistance and weather resistance equal to two-layer coating film.
Japanese Patent Kokai Publication No. 281943/2000 describes an electrodeposition coating composition containing a binder component dispersed in neutralizing agent-containing aqueous solution and a pigment, wherein the binder component contains two acrylic resins having independent solubility parameter.
The aforementioned inventions described in Japanese Patent Kokoku Publications Nos. 33069/1990 and 99652/1994 and Japanese Patent Kokai Publication Nos. 333528/1996 and 292131/1998 merely show a technique of layer (resin) separation in electrodeposition coating. These inventions hardly provide a multi layered coating film having excellent appearance and weather-resistance equal to the conventional three-coat coating film.
The composition described in Japanese Patent Kokoku Publication No. 33069/1990 contains a cationic phenol type epoxy resin having a softening point of 75° C. or less, which however adversely affects on a resulting cured film in solvent resistance and corrosion resistance. On the other hand, in Japanese Patent Kokoku Publication No. 99652/1994, a nonionic film-forming resin used may provide inferior adhesiveness to top coating compared to an ionic film-forming resin such as a cationic resin. Therefore, a new solution of these problems has been required.
Based on the process described in Japanese Patent Kokai Publication No. 345394/2000, it is possible to form an electrodeposition cured coating film with complete layer separation based on inclusion of two different resin components having solubility parameter difference of not less than 1. However, when a solvent-type top coating was applied over the forming cured multi layered coating film, the cured coating does not have sufficient solvent resistance, even if an appropriate curing agent is formulated into electrodeposit composition with the aim of improvement of solvent resistance (i.e. lowering a solvent swelling degree of cured multi layered film). When solvent-type top coating is applied on the cured multi layered coating film with insufficient solvent resistance, the solvent penetrates into the coating film, and the fluidity of top coating composition disappears so that the multi layered coating film would show poor appearance. In addition, ply separation problem due to long term weather deterioration may occur, which may be caused by clear layer separation in multi layers has found. Thus, a method for forming a cured multi layered coating film with good ply-separation resistance on long term weathering is desired.